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EARTHQUAKE FIELD GUIDE Spotting Common Problems in Bay Area Housing

Association of Bay Area GovernmentsAugust 2018

The Association of Bay Area Governments and

the Metropolitan Transportation Commission

are the regional land use and transportation planning agencies

for the nine counties and 101 cities and towns

of the San Francisco Bay region.

ABAG’s Resilience Program

assists Bay Area local governments,

and provides resources for residents in planning for earthquakes,

the effects of climate change and other hazards.

The Field Guide was generously funded by

grant G16AP00172 from the US Geological Survey.

2018

EARTHQUAKE FIELD GUIDE Spotting Common Problems in Bay Area Housing

Association of Bay Area Governments, 2018

Table of ContentsIntroduction 1

Bay Area Earthquake Risk 2

Building Types 5

Single Family Homes 7

Home With a Crawl Space 8

Home With a Slab Foundation 10

Living Space Over Garage Homes 12

Home Built Above Steep Hillside 14

Home Built Into a Steep Hillside 16

Split Level Home 18

Mobile or Manufactured Homes 21

Multi Family Homes 25

Wood Frame With Soft, Weak Story or Open Front 26

Wood Frame, Without Soft, Weak Story or Open Front 28

Unreinforced Masonry or Stone 30

Other Multi Family Buildings 32

Glossary 33

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Spotting Common Problems in Bay Area Housing 1

IntroductionLiving in Earthquake Country means that Bay Area residents need

to be prepared for an earthquake and possible damage to their

homes, ranging from cracks and leaning to complete collapse. When

housing is damaged, people and communities are impacted in many

ways. People in or near their homes may be harmed. Many people

will have to move out of their homes, depending on shelters or

temporary housing, for weeks, months, or even years as their homes

are repaired or rebuilt. Homeowners may no longer be able to

afford their mortgages, or find repairs too expensive, and may move

away for good. Renters may not be able to find another place to live

in a hyper-competitive market. Even homeowners and renters with

an earthquake insurance policy will struggle to rebuild their lives if

their homes are seriously damaged.

Use this Field Guide with the Online Home Quiz to Identify Risks in Your Home or Rental.The Earthquake Field Guide: Spotting Common Problems in

Bay Area Housing (Field Guide) is designed to help Bay Area

residents identify possible earthquake-related problems in

common home types in the region. The Field Guide only

identif ies potential problems with a home’s structure. The

Field Guide should be used in conjunction with the online

Earthquake Home Quiz (Home Quiz) to identify additional

potential problems, including geologic hazards, non-structural

items like furniture, chimneys and water heaters. The Home

Quiz (homequakequiz.org) provides residents with next

steps to improve earthquake performance.

Only an engineer can tell you for sure if a home is vulnerable.

The Field Guide and Home Quiz are a starting point.

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2 Earthquake Field Guide Spotting Common Problems in Bay Area Housing 3

Bay Area Earthquake RiskThe Bay Area is riddled with earthquake faults capable of producing

both small and large earthquakes. In the next 30 years, scientists

predict there is a 72 percent chance of a magnitude 6.7 or greater

earthquake on at least one of these faults. Quakes of this magnitude

could cause violent shaking anywhere in the Bay Area. Figure 1

below shows how an earthquake on any number of active faults in

the region could distribute shaking, which helps to predict how

many housing units may be damaged in that particular earthquake

scenario.

Figure 1: 16 Potential Earthquake Shaking Maps and Estimated Households Displaced by Damage (Estimated households displaced shown in parentheses.)


Earthquakes are common in the Bay Area, and the region has a

history of damaging quakes. In 1868, an earthquake estimated at

a magnitude of about 7.0 occurred on the Hayward Fault. Perhaps

most famously, the magnitude 7.8 1906 earthquake on the San

Andreas fault devastated San Francisco and caused extensive

damage all the way from San Jose up to Santa Rosa. Twenty-two

magnitude 6.0 or greater events have rocked the Bay Area since

1840, and smaller earthquakes occur every day. The 2014 South

Napa earthquake was a wake-up call, reminding residents that even

moderate earthquakes can damage homes and disrupt lives.

Building TypesThe Bay Area is home to a wide range of housing types, from

different eras, and in different styles, ranging from the ornate

Victorians of San Francisco, to East Bay craftmans, colorful hillside

homes, mid-century ranches, high-rise apartments, and everything

in between. Every type of home in the region may be vulnerable to

earthquakes in different ways, depending on how it was built and

how long ago. While many variations on these homes exist, some

common types are widely identified throughout the region.

This Field Guide distinguishes eleven common Bay Area housing

types. It is intended to help you identify your home’s type

through easy visual cues, and understand possible earthquake

problems unique to that type. From there, you can take our online

Earthquake Home Quiz to dig deeper into specific vulnerabilities

and find out what you can do to improve your home’s performance

in an earthquake.

Lastly, only an engineer can tell you for sure if your home is

vulnerable. This Field Guide, and the Earthquake Home Quiz, are

a starting point. If you are unsure if your home may be vulnerable,

call a contractor or engineer to get a professional opinion.

A magnitude 7.0 earthquake releases 33 times more energy than a magnitude 6.0 and 1000 times more than a magnitude 5.0 earthquake. The M7.8 1906 earthquake released 500 times more energy than the M6.0 2014 South Napa Earthquake (sphere volume is proportional to quake energy).

6.9

M7.8

6.0

1906 M 7.8The “Big One” 1989 M 6.9

Loma Prieta

2014 M 6.0South Napa

Over this 58 year period the

Bay Area did not experience a

magnitude 6.0or greater

earthquake.

There is a72%

probability of a M 6.7or greaterBay Area

earthquakein the next30 years.

1830 1850 1950 1970 1990 2010 20301870 1890 1910 1930

Figure 2: Earthquake Magnitudes and History of Magnitude 6+ Bay Area Earthquakes


Single Family HomesBay Area single family homes come in a variety of types and

styles. Many neighborhoods were built around the same time, so

the houses have similar materials and designs. The overwhelming

majority of single family homes are wood frame construction. In

small quakes, wood frame houses tend to do well, because wood

can bend and twist a little without breaking. However, in stronger

shaking, the damage may be severe and is described for each home

type in the following pages.

Many identifying features are relatively distinctive and will

make identification easy, while others may be more difficult.

Additionally, your home may be a blend of multiple types. If your

home has characteristics of more than one type, explore the sections

for each type.

Use this Field Guide to identify what possible problems your home

may have, and take the Home Quiz for more detailed information

about potential problems and a list of things you can do to avoid

damage and make your property safer.


Home With a Crawl Space Also Called: Cripple Wall

One of the most common types of single family homes in the Bay

area is wood frame with a crawl space. The term crawl space refers

to unoccupied space between the foundation and the first f loor,

which is usually only a few feet tall, and is framed by a “cripple

wall.” Most single family homes on f lat sites built before 1940 have

a crawl space, as well as both older and newer homes built on slight

slopes.

Identifying Features

●1 The first f loor is not level with the ground.

●2 There are typically multiple steps up to the doors.

●3 There are typically vents/screens along the perimeter of the

house between the foundation and the first f loor.

●4 There is often a larger opening that allows for someone to access

the crawl space.

Common Problems in an EarthquakeIf the cripple wall frame is not bolted to the foundation, the home

may slide off the foundation. If the cripple walls are not strong

enough, some or all of the crawl space can collapse under the weight

of the home above. If either of these occur, the home can sustain

considerable damage, and may be uninhabitable. In addition, the

home’s gas, water or sewer lines may also break. The cost to repair,

or demolish and rebuild, can be very high.

What to do NextGo to homequakequiz.org and answer a few more questions

about your home. After that, you’ll be given specific next steps and

resources to help you address potential problems.

This home may also have• Living Space Over Garage – p. 12• Built Into a Steep Hillside – p. 16• Split Level – p. 18

●1

●2

Multiple steps up to doors

First floor is not level with

the ground

●1

●2

●4First floor is not level with

the ground

Multiple steps up to doors Larger opening

allowing access to crawl space

Vents along house

perimeter

Larger opening allowing access to

crawl space

●4

●3Vents along

house perimeter

●3


Home With a Slab Foundation Also Called: Slab on Grade

Many homes built after the 1940s on f lat sites are built with a slab

foundation. A slab foundation is made of a solid slab of concrete and

extends to all sides of the home. The home typically sits directly on

top of the slab.


●1 The first f loor is level or near-level with the ground.

l2 There is typically zero to one step to the front door.

l3 There are typically not vents present, however, there may be a

single appliance vent near the ground on the first f loor.

Common Problems in an EarthquakeIn an earthquake, structures with this foundation type generally

perform well. In some cases, pre-1960 homes on a slab foundation

may not be sufficiently anchored to the foundation and can shift in

an earthquake. Some walls of the home may lean and crack in an

earthquake if this happens.




This home may also have• Living Space Over Garage – p. 12

●1

●1

●2

●2

●3

●3

First floor even with ground

Zero or one step to front

door

Zero or one step to front

door

No vents near

ground

No vents near ground

First floor even with ground


Living Space Over Garage Homes Also Called: House or Room Over a Garage

There are two versions of homes with living space above a garage.

House Over Garage, where the first story of the home primarily

consists of a garage and unfinished space (i.e. not livable space)

and the majority of the living space is above this,

Room Over Garage, where only a portion of the house is above the

garage and the rest of the house has a different foundation type

(i.e. crawl space or slab foundation).


l1 The garage is integrated into the structure of the house, with

living space above.

l1a House Over Garage, all living space is on the second f loor

above the garage.

l1b Room Over Garage, some living space is on the first f loor, some

is above a garage.

Common Problems in an EarthquakeIn an earthquake, the garage or unfinished space on the first f loor

may not be strong enough to resist the side-to-side shaking of an

earthquake, leading to collapse of the garage area, or causing a

permanent lean. If the garage collapses, the second f loor area above

it is also likely to be a greater risk for serious harm to occupants in

this portion of the home. The home can become uninhabitable if the

garage wing or first story leans significantly or collapses.




This home may also have• Crawl Space – p. 8• Split Level – p. 18

●1

●1

●1

House Over Garage Entire house is above garage space

Room Over Garage Portion of home is above garage

l1a l1a

l1b


Home Built Above Steep Hillside Also Called: Down-Slope Hillside Home

Homes built above a steep hillside have increased potential for

earthquake damage. In these homes, the main living level is built at

or near street level, with piers or posts embedded in the ground that

support the rear of the house. Homes built into steep hillsides are

discussed on the next page.


l1 If slope drops one or more feet for every five horizontal feet on

the perimeter of the building it is a “steep” hillside.

l2 The home extends out over the hillside with living or

unfinished space below, either supported by tall stilts (may be

braced, see image), or tall walls.

Common Problems in an EarthquakeIn an earthquake, the home can shift, collapse downhill or tilt

sideways on the hill. The anchorage connection to the uphill

footing is a common weak point and the bracing or tall walls are

also often not strong or stiff enough to resist the shaking. If the

building collapses, residents can be injured or killed.




1 ft

5 ft

1 ft

5 ft

1 ft

5 ft

1 ft

5 ft

1 ft

5 ft

1 ft

5 ft●1

●1

●2

●2

Grade changes one foot or more for every five horizontal feet


Home above tall stilts with

unfinished space below

Home above tall walls

with living or unfinished

space below


Home Built Into a Steep Hillside Also Called: Up-Slope Hillside Home

These homes typically perform better than their Home Built Above

a Steep Hillside counterparts (described on the previous page), but

often have at least one of three other potential problems: crawl

space, living space over garage and/or split level.


l1 If slope drops one or more feet for every five horizontal feet on

the perimeter of the building it is a “steep” hillside.

l2 Portions of rooms in the home are built into the hillside. The

home was built into excavations cut in the slope.

Common Problems in an EarthquakeIn an earthquake, damage is typically related to the other problems:

crawl space, living space over garage or split level. If your home has

any of these problems visit these sections for common issues in an

earthquake.




1 ft

5 ft

1 ft

5 ft

1 ft

5 ft

This home may also have• Crawl Space – p. 8• Living Space Over Garage – p. 12• Split Level – p. 18

●1 ●2

Rooms in the home are built into the hillside



Split Level HomeSplit level homes come in a variety of shapes and sizes. The split

level can be an architectural feature or be a result of sloping ground,

or a garage with a living space above. Sometimes a split level home

can be identified from the outside, but often verifying features are

needed on the inside – mainly steps between rooms that don’t go up

or down a full story.


l1 Floors that are at different levels, but less than one full story

difference.

l2 Roof lines that are at different levels, but less than one full story

difference.

Common Problems in an EarthquakeIn an earthquake, the home may respond as two separate sections,

one on each side of the shared wall. Separation between the sections

can occur at the shared wall, with the possibility of severe damage

or partial collapse. If the split level home also has a living space over

garage or a crawl space, these elements can also fail as described in

other sections.




This home may also have• Crawl Space – p. 8• Living Space Over Garage – p. 12• Built Into a Steep Hillside – p. 16

●2

Roof lines differ by more than

half story

Floors at different

elevations

●1


Mobile or Manufactured HomesMobile and manufactured home communities have experienced

higher concentrations of damage in past California earthquakes.

After earthquakes, many mobile homes required lifting and setting

the mobile homes into their original positions, while others were

unrepairable. In the 1994 Northridge earthquake in Southern

California, half of the mobile homes in areas of strong shaking

fell off of their supports, and an additional 10% had other forms

of damage. In the 2014 South Napa earthquake, in certain mobile

home communities, 25% of homes shifted off of their supports.






Mobile or Manufactured HomeMobile and manufactured housing is found in pockets across

the Bay Area. The walls of mobile homes are fairly seismically

resilient because they are designed to resist high wind loads during

transportation, but the homes are often highly vulnerable because

many do not have a proper bracing system below the f loor or a

proper foundation.


l1 The homes are often in a neighborhood specifically designated

for mobile or manufactured housing.

l2 The homes are often placed on concrete blocks or steel piers

2-4 feet above the ground.

l3 Most manufactured and mobile homes have a steel chassis under

the first f loor, enclosed by covering panels of aluminum or

plywood.

Common Problems in an EarthquakeIn an earthquake, these homes can fall off their supports, damaging

the home and rupturing utility connections (gas, water, sewer)

which can cause fires.




Mobile Home Estates

●1

●2

●2

●3

●3

In a mobile home community

Steel chassis under first floor

Steel chassis under first floor

On blocks 2-4 feet above

the ground

On blocks 2-4 feet

above the ground


Multi-Family HomesMulti-family homes in the Bay Area are built in a wide variety

of types. They can be apartments or condominiums, sometimes

with small businesses or parking on the first story. Similar to single

family homes, there are potential seismic problems in many types

of multi-family buildings.

The focus of this guide is on wood frame multi-family housing,

typically fewer than 5 stories. Multi-family homes of all heights can

also be built of concrete or steel. Older concrete and steel buildings

can be vulnerable to damage in earthquakes but are not addressed

here; see page 32.

Some smaller (duplex, triplex and quadplex) buildings might not

match a building type in this section but may be more similar to a

type described in the Single Family Homes section.






Wood Frame With Soft, Weak Story or Open Front Also Called: Soft Story

Many wood frame apartment and condominium buildings have

openings on the ground f loor to allow for parking or storefronts.

Buildings of this type built before 1978 may be unable to resist

earthquake shaking in the bottom level, and even some built before

1995 may have potential problems. Some Bay Area cities have

created lists of potential soft story buildings that residents can use to

look up addresses, with some cities even requiring retrofit.


l1 First story consists primarily of large openings (garage doors,

garage spaces, large storefront windows) with mostly residential

units above.

l2 The ground f loor is constructed with wood frame.

l* If your building is constructed with concrete, or cinder blocks

on the first story it is not a wood frame soft story building. It is

likely a “podium building” (not shown in this guide).

Common Problems in an EarthquakeThe ground f loor has more large openings, and fewer walls than the

f loors above it, so it is less able than upper stories to resist shaking.

In an earthquake, the lower story can tilt sideways or collapse,

resulting in total loss of the building and a significant risk to life for

occupants. If the building does not collapse it may tilt permanently,

which would displace tenants.




OPEN

OPEN

OPEN

OPEN

Large openings on ground floor: car opening, storefront windows

●1

Large openings on ground floor: car opening●1


Wood Frame Without Soft, Weak Story or Open FrontThere are many wood frame multi-family buildings that do not

have any large openings on the first story. Many are two stories, but

they could be as high as five stories. The walls may be covered by

wood siding, stucco or brick veneer. In earthquakes, these buildings

tend to perform well - problems occur if the building also has a

crawl space below the first story.


l1 Wood frame construction is typically less than five stories high.

l2 There are no large openings (garage entrance, wide storefront

windows) in the walls.

Common Problems in an EarthquakeIn an earthquake problems may occur if the foundation is a crawl

space or slab foundation. If the base is a crawl space, the building

can fall off the foundation. If the building is on a slab foundation it

may shift and lean in an earthquake. For more detail on problems in

an earthquake look at Crawl Space or Slab Foundation in the Single

Family Home Section.




●2

●2

This home may also have• Crawl Space – p. 8• Slab Foundation – p. 10

No large openings (e.g., garage entrance or wide storefront windows)

in the walls

Wood frame construction is typically less than five stories high

Wood frame construction is typically less than five stories high

No large openings (e.g., garage entrance or wide storefront windows)

in the walls

●1

●1


Unreinforced Masonry or Stone Also Called: URM

While relatively uncommon as a Bay Area housing type, residential

buildings built exclusively with brick or stone (unreinforced

masonry) are some of the most dangerous buildings in earthquakes.

These types of buildings rely on bricks or stone and mortar for

their structure, and do not have a separate wood or steel frame.

In California, a building code change in 1933 eliminated the

construction of most new unreinforced masonry buildings. Most

Bay Area cities have identified their URM buildings. Many cities

have mandated strengthening, but some have not.


l1 Have walls made out of brick or stone. Bricks may be visible on

both the outside and inside walls.

l2 If every 5th or 6th row of bricks has a different width (because

it has been turned perpendicular), this indicates that the wall is a

likely a structural brick wall, not just a decorative siding.

l3 May have thick window sills due to thicker walls (more

common in stone buildings, but may also be seen in brick

buildings).

Common Problems in an EarthquakeIn an earthquake, the entire building or individual building

components (sections of walls above roof line, portions of walls) can

collapse, killing or injuring people inside and around the perimeter

of the building. The first component to fail is often the bricks

at the top of the building that extend above the roof. Walls can

collapse inward or outward onto streets, sidewalks, or neighboring

buildings. The roof can collapse inward.




●3

●2

●1Walls are brick or stone

Thick walls

Every 5th or 6th row of bricks is

different width


Other Multi-Family Building Types not IncludedThere are other multi-family building types in the region that

also have potential seismic problems. Concrete and steel buildings

are not covered in this document because they can be difficult to

identify visually without an engineer. Most structures taller than

five stories are concrete or steel frame construction, but these

building types may also exist for buildings less than five stories.

Common Problems in an EarthquakeThere are many different construction methods for concrete and

steel buildings. Many perform well in earthquakes, while others are

known to be a risk for collapse or experience irreparable damage,

especially if they are older buildings. At this date, we do not have

good simple guidance for non-engineers to identify deficiencies in

concrete and steel buildings.

What to do NextThe best approach for these buildings is to have a licensed engineer

perform a preliminary seismic evaluation of likely problems. You

can go to homequakequiz.org and answer questions about

mapped geologic site conditions.

GlossaryAnchorage – Hardware used to connect parts of structures together

and to the foundation, such as, nails, sill plates or bolts.

Building Code – Set of rules that specify the standards for building

construction, updated every few years. In California

the building code has improved over time as more

knowledge of earthquake-resistant design has been

developed. Buildings built to older codes are typically less

earthquake-resistant.

Chassis – Manufactured housing is often built on top of a steel

chassis which is a rigid base, allowing for transportation of

the manufactured housing.

Concrete and Steel Frame – The frame of a building gives a

structure support and shape. The structural support

elements in concrete or steel frame buildings are made

with concrete or steel. Large residential buildings, or

buildings designed for commercial, retail or office uses

are often built with concrete and steel frame elements.

These buildings may be difficult to identify without an

engineering background or access to building plans.

Cripple Wall – A wood wall that encloses the crawl space (unoccupied

space) under the first f loor of a wood frame home.

Footing – A footing is a concrete or masonry foundation element

that sits on the soil and forms the base on which a home

is constructed. Footings at a home perimeter are often

continuous strips of material.


Inhabited – Spaces of a building that are designed to be occupied

and used by people. Uses of inhabited spaces include living

areas, offices, shops and parking.

Magnitude (M) – A number that characterizes the relative energy

release of an earthquake. Magnitude is measured on a

logrithmic scale, which means a magnitude 7.0 earthquake

is 33 times more powerful than a 6.0 earthquake.

Modified Mercalli Intensity (MMI) – Estimates the intensity of

shaking from an earthquake at a specific location by

considering its effects on people, objects and buildings.

The scale runs from 1 to 12. Poor performing buildings

can be damaged in MMI 7, with some collapsing in MMI

8 and many collapsing in MMI 9. Well-constructed

buildings may also be damaged in MMI 9 shaking

intensity.

Partially below grade – An area of a building that is built below

the ground level along the surrounding exterior walls.

Piers or Posts – Vertical members between the ground and first

f loor of a building. They support the home vertically, but

are often not designed to support a building that is shaken

sideways in an earthquake.

Podium Building – In a multi-family residential building, a ground

story constructed of concrete or masonry while the stories

above are wood frame. The ground story is often used for

parking or shops.

Possible or Potential – The field guide generalizes common

problems found in Bay Area housing. In this guide, the

word “possible” or “potential” means something might

happen but is not for certain.

Safety Risk – In earthquakes, building failures can cause injury

or death to inhabitants. Some earthquake damage to

buildings doesn’t present a safety risk to humans, but other

damage can injure and in some cases be fatal. The online

Quiz addresses safety risks from nonstructural elements

like furniture and appliances.

Soft Story – Describes buildings that lack sufficient bracing to

resist earthquake shaking. Soft story buildings are often

identifiable by the presence of large openings for windows

or parking stalls in the building’s perimeter walls. Also

called “wood frame with soft, weak story or open front.”

Unreinforced Masonry (URM) – Brick or stone wall construction

that does not have embedded steel bars (reinforcing).

Newer masonry construction has embedded steel bars

that strengthens it for earthquakes. Embedded bars were

not included in older masonry construction, making it

vulnerable to earthquake damage.

Unfinished – In many homes the first f loor of a building, the

garage, or the portion of a home built into a hillside

remains unfinished. Unfinished spaces often have exposed

interior walls that lack interior drywall or plywood, and

can be less resistant to earthquakes than finished spaces.


Uninhabited – A portion of a building that is not designed for

people to spend time. Examples of common uninhabited

space are crawl spaces and other unfinished areas.

Veneer – A finish material applied to the outside of a wall, most

often made of brick or stone.

Wood Frame – The frame of a building gives a structure support

and shape. The interior walls in wood frame buildings are

made with wood. Because the wood frame is covered by

a range of materials (plywood, wood siding, stucco and

drywall) it can be difficult to know that it is wood frame.

Most single family homes in the Bay Area are wood frame

buildings, and most residential multi-family structures less

than 5 stories are also wood frame.

Credits

ABAG and MTC Staff

Michael Germeraad Resilience Planner Principal Author

Dana Brechwald Resilience Planner Co-Author

David Cooper Graphic Designer Line Art Graphics

Peter Beeler Lead Graphic Designer Document Layout

Leah Zippert Senior Public Information Officer Editor

Technical Advisory Committee

David Bonowitz Structural Engineer

Kelly Cobeen Associate Principal Structural Engineer,

Wiss Janney Elstner Assciates, Inc.

Curt Haselton Professor of Civil Engineering,

California State University Chico

Janiele Maffei Chief Mitigation Officer,

California Earthquake Authority

Marko Schotanus Senior Associate Structural Engineer,

Rutherford & Chekene

Fred Turner Staff Structural Engineer,

California State Seismic Safety Commission

Resilience Program

375 Beale Street, Suite 700, San Francisco, CA 94105

415.820.7900

resilience.abag.ca.gov

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